As it is well known, the exhaust gases of these engines contain many pollutants, such as unburnt hydrocarbons, carbon monoxide, nitrogen oxides (NO and NO2), more commonly referred to as NOx, for engines running on gasoline or gas, and additionally particles for Diesel type engines.
In order to comply with emission standards and to preserve the environment, these pollutants have to be treated prior to discharging the exhaust gases into the atmosphere.
As it is generally known, this is achieved by means of a system for treating the exhaust gas circulating in the exhaust line of the engine.
Thus, catalysis means, more commonly referred to as oxidation catalysts, allow to oxidize the unburnt hydrocarbons and the carbon monoxide for engines running with a lean mixture.
For a Diesel engine exhaust line, a particulate filter can be arranged on this line in order to retain the particulates present in the exhaust gas and thus to prevent them from being discharged into the atmosphere.
This filter, which can also be a catalyzed filter, has to be periodically regenerated in order to keep all its filtration capacities. The regeneration operations mainly consist in increasing the temperature of the filter, generally by increasing the richness of the exhaust gases flowing therethrough. The higher temperature that is obtained allows to carry out combustion of the particulate matter retained in this filter.
Furthermore, in order to allow NOx treatment, the exhaust gases also flow through other catalysis means, notably SCR (Selective Catalytic Reduction) catalyst type catalysts. This SCR catalyst allows to selectively reduce the NOx to nitrogen through the action of a reducing agent.
This agent, generally injected upstream from the SCR catalyst, can be ammonia or a compound generating ammonia through decomposition, such as urea, or a hydrocarbon from a hydrocarbon-containing substance.
This reducing agent mixes with the exhaust gases, then it reacts with the NOx of the exhaust gases on the SCR catalyst according to several possible chemical reactions such as, for example:4NH3+2NO+2NO2→4N2+6H2O or 4NH3+4NO+O2→4N2+6H2O.
The precursor of the reducing agent conventionally used for this application has to be stored in a large volume, whereas the space available in the vehicle is limited. A large volume is necessary because this precursor is stored in liquid form and diluted in water.
Besides, SCR catalysts, which use ammonia as the NOx reducing agent, are generally active only above a light-off temperature ranging between 180° C. and 250° C. depending on the nature of the catalyst. Below this temperature, and in particular upon cold start of the vehicle, the NOx are not treated by ammonia-based SCR catalysts.
The present invention allows to overcome the aforementioned drawbacks by using a compound of high density or stored in solid form. This allows the compound to be arranged in the vehicle with a restricted volume.